This invention relates to a method of producing a thin-film electroluminescent device in which the electroluminescent phosphor layer is a thin film of zinc sulfide doped with manganese (ZnS:Mn), the method using a chemical vapor deposition process to form the ZnS:Mn film.
In place of electroluminescent (EL) devices having an active layer formed of a dispersion of EL phosphor particles in plastic or ceramic, recently much attention has been directed to thin-film EL devices having a thin film of an EL phosphor as the active layer. As the EL phosphor, ZnS:Mn is prevailing.
In producing thin-film EL devices usually the phosphor film is formed by a physical vapor deposition technique such as vacuum evaporation by electron-beam heating or sputtering. In practice, however, there are many problems which are difficult to solve completely. In the case of vacuum evaporation by electron-beam heating the problems include ejection and scattering of clusters of the evaporating material from the crucible as a cause of surface defects (1-10 .mu.m in diameters) in the deposited film high probabilities of pinholes in the deposited film and unevenness of the deposited film as an inevitable result of evaporating the phosphor material substantially in one point. In the case of sputtering the problems include damaging of the deposited film by the secondary electrons emitted from the target, intrusion of the sputtering gas into the deposited film and changes in the chemical composition of the target surface during the sputtering operation.
There is a proposal of depositing a ZnS:Mn film by a so-called metal-organic chemcial vapor deposition (MOCVD) process using a mixed gas containing an organic zinc compound such as dimethylzinc or diethylzinc, an organic sulfur compound such as dimethyl suflfide, diethyl sulfide or hydrogen sulfide and an organic manganese compound (e.g. JP-A No. 62-98596). However, when dimethyl sulfide or diethyl sulfide is used it is necessary to keep the substrate heated at a temperature about 500.degree. C. for accomplishing CVD, and even though the substrate temperature is so high the rate of growth of a ZnS:Mn film is very low. When hydrogen sulfide is used there occurs a vapor phase reaction between the sulfide and the organic zinc compound to form solid particles of zinc sulfide precedent to the decomposition of the gaseous compounds on the substrate surface, and it is inevitable that the solid particles adversely influence the quality of the deposited film.